My name is Sue Pinkney. I am a descendant of the GAFF's from Cawston. There is a group of us researching these GAFF's and it's great to hear of another one. You are the first of John and Honoria's family to make contact.
I have a complete set of indexes to the Cawston Parish Registers and the lovely lady who did them is happy to do lookups. She has also send me the complete GAFF entries from the registers. Though it does seem like you have most of the information.
My belief is that there were two brothers - Jonathon and John, which is most confusing. I have not followed down John's line but have just tried to keep to Jonathon's as that is my line and not really knowing the relationship between the two. The first mention in the Cawston registers is Jonathon's marriage to Mary GROOM in 1776.
This was followed by the marriage of John GAFF and Honoria OUTLEY in 1783.
The fact that Jonathon named two children - brothers who both survived and married and had children of their own - John and Jonathon, makes me think that it was not unusual and both names were independent of each other. Now we would probably consider them the same name.
Where Jonathon and John came from I don't know. I think it is fairly certain that they were not locals as the cleric took a stab at the spelling of the name GARPH and maybe they had an accent. There is a fairly big grouping of GAFF's in Guildford in Surrey and there seems to be a large GAFF clan in the Stirling area of Scotland.
As to the question of memorial stone inscriptions. I was in Cawston in August last year and there are no stones for any GAFF's. The GAFF's were Ag Labs and were poor. They were most likely buried in the pauper's graves around the edges of the graveyard. We also spoke to the vicar of St Agnes and he said that there was a mass grave to one side of the church where the poor of the parish were buried. So unfortunately if you are unable to unravel what you want from the parish records, I don't think you will find the info anywhere else.
You can find the Cawston Parish indexes online at http://www.genealogy.doun.org/transcriptions/index.php3
You can search by surname or by parish. If you want something in full you can ask the source or I can help with the GAFF entries.
I would love to hear from you and can be contacted at firstname.lastname@example.org